In transmission systems it is often desired to transmit in addition to a main signal an auxiliary signal. The main signal can e.g. be a speech signal to be transmitted via a radio link of a mobile telephone system. The auxiliary signal can be e.g. a control signal for requesting a reconfiguration of a the receiver to adapt it to a change of the signal it should receive.
It is possible to include a special field in the transmission frame for such auxiliary signal, but this inclusion of a special field in the transmission frame is very inefficient if the auxiliary signal is rarely used. It is also possible to use a frame structure which is changed when the auxiliary signal has to be transmitted. The use a variable frame structure results in a substantial increase of the complexity of the transmission systems.